In the manufacture and assembly of rotary dynamoelectric machines, such as electric motors or generators, establishing and maintaining a concentric relationship between the rotor and stator is essential for the smooth and efficient operation of such machines. End caps of a casing, also sometimes referred to as end plates or bearing shields, seat a rotor assembly (rotor, drive shaft and bearings). Piloting of end caps to a stator's inner diameter has proved to be the best approach to establishing proper centering and axial alignment of the rotor within the stator. However, traditional end cap designs have required precision machining to very tight tolerances.
FIGS. 1a-1h illustrate this precision machining for a typical motor end cap. FIGS. 1a and 1b show different assembled motors, one with a square perimeter end cap with screw holes in the corners, and the other with a circular perimeter end cap that is form fitted to the rest of the motor casing. Thus, the exterior portion of an end cap does not normally require the precision machining that is demanded of certain interior portions of the end cap, namely of the piloting ring 11 seen in FIG. 1d that engages with the stator assembly. The traditional end cap is made of a metal such as aluminum. This metal end cap has a central opening 13 through which passes a drive shaft for the rotor assembly. The piloting ring 11 has an inner diameter 15 that closely matches a bearing outer diameter so that the bearing will just fit within the piloting ring 11. The piloting ring 11 also has an outer diameter 17 that closely matches an inner diameter of a stator's plastic (insulating) winding frame for a tight fit. The winding frame's insulator plastic inner diameter is honed in-line with the stator inner diameter. Typical dimensions are 6 mm diameter for the central opening, 16 mm diameter for the piloting ring inner diameter, and 22 mm diameter for the piloting ring outer diameter. The tolerances are very tight for the piloting ring diameters and their concentricity with each other, generally to within 10 μm and preferably 5 μm).
Because of the cost incurred in the precision machining of the end caps, an end cap wherein the tolerances can be relieved without adversely affecting motor quality and performance would be beneficial.